As a photographic printer that records images based on image data, a laser printer using laser beams is known, wherein the laser beams is scanned across a sheet of photosensitive material in a main scan direction as the photosensitive material is conveyed in a sub scan direction orthogonal to the main scan direction. An example of the photosensitive material is of a silver-salt type, on which a latent image is recorded by the scanning-exposure, and comes up to the surface by photographic development.
The laser printer is provided with a scanning-exposure apparatus that uses a light source device for generating laser beams of three primary colors: red, green and blue. The laser beams are modulated in accordance with corresponding color image data. The modulated laser beam of each color is refracted in the main scan direction by use of a polygonal mirror, to scan and expose the photosensitive material while it is being conveyed.
In such a scanning-exposure apparatus, light sources for the red light beam and the blue light beam are generally semiconductor lasers or laser diodes, whereas a light source for the green light beam is a second harmonic generation (SHG) laser that is a combination of a diode-pumped solid-state laser and an SHG element.
The semiconductor laser for red is generally configured by forming a light emitter on a substrate made of GaAs, and the light emitter has a double-hetero structure where an active layer is sandwiched between a light guide layer and a cladding layer. The light emitter uses AlGaIP, AlGaAs and InGaAsP as its materials. The GaAs substrate absorbs light components of its oscillation wavelength. Since the opposite electrode uses a light absorbing material like InGaAs, the light emitted from the red semiconductor laser is confined to an emission range of several micrometer wide. Therefore, the red semiconductor laser does not specifically suffer from stray light beams that are caused by other semiconductor materials than the light emitter.
On the other hand, the semiconductor laser for blue generally uses materials of GaN group. Since a single crystal of GaN has not yet been put into practice, the blue semiconductor laser uses a substrate made of such a substitute material that is transparent to light components of the oscillation wavelength, as sapphire (Al2O5) and SiC.
As shown in FIG. 11, an GaN group semiconductor laser is mainly constituted of a sapphire substrate 42, a buffer layer 41 formed from n-GaN on the sapphire substrate 42, a light emitter 40 formed on the buffer layer 41 and has a double-hetero structure, a positive electrode 50 connected to a top of the light emitter 40, and a negative electrode 51 connected to the buffer layer 41. Applying a voltage across the positive and negative electrodes 50 and 51 to raise current injected into the light emitter 40 causes the laser emission to project the blue laser beams from an active layer 45 of the light emitter 40 in a direction substantially perpendicular to the drawing surface of FIG. 11.
At that time, the blue laser beams from the light emitter 40 are partially diffused as stray lights inside the sapphire substrate 42. Some of the stray lights come back to the light emitter 40, and some leak outsides. The stray light leaked in the projecting direction of the sapphire 42 is called flare, and is transmitted along with the true blue laser beams that are projected directly from the active layer of the light emitter 40. So the flare badly affects on the shape of beam spots focused on the photosensitive material, damaging the quality of the recorded image.
In order to eliminate the flare, a light source device disclosed in Japanese Laid-open Patent Application No. 2001-24230 suggests placing a slit plate or a pinhole near a focal position of a condenser lens. A scanning-exposure apparatus disclosed in Japanese Laid-open Patent Application No. 2003-255253 suggests eliminating the flare by limiting numerical aperture of an aperture that is provided at an entrance of a collimator lens. Japanese Laid-open Patent Application No. 2000-66128 suggests a scanning-exposure apparatus that eliminates the flare by providing a pinhole between a light source and a collimator lens.
Although above three prior arts can eliminate the flare to some extent by limiting the light flux of the laser, these prior arts cannot efficiently eliminate the flare from the laser because the study for defining an optimum position of the slit plate or the pinhole has not sufficiently been done in this field.